U.S. Pat. No. 4,392,171 for a "Power Relay With Assisted Commutation"--issued July 5, 1983--William P. Kornrumpf, inventor and assigned to the General Electric Company, discloses an electromagnetic (EM) relay with assisted commutation wherein the load current carrying contacts of the relay are shunted by a gatable semiconductor device that assists in commutation of contact destroying arcs normally produced upon closure and opening of such contacts. This device is typical of AC power switching systems which employ a parallel-connected semiconductor device connected across a set of current interrupting power switch contacts for temporarily diverting the current being interrupted during opening or closure of the contacts. After current interruption and with the relay contacts opened, there still exists a high resistance current leakage path through the parallel connected gatable semiconductor device in its off condition due to the inherent characteristics of the semiconductor device. Underwriter Labs (U.L.) has decreed that such switching circuits are not satisfactory for use with home appliances and other similar apparatus due to the prospective danger of the high resistance current leakage paths electrically charging the home appliance or other apparatus to a high electric potential that could prove injurious or lethal or otherwise fail in a non-safe manner.
U.S. Pat. No. 4,296,449, issued Oct. 20, 1981 for a "Relay Switching Apparatus"--C. W. Eichelberger--inventor, assigned to the General Electric Company, discloses an AC power switching circuit that employs a diode commutated master electromagnetic operated relay in conjunction with a pilot EM operated relay with the switch contacts of the master and pilot relays being connected in series circuit relationship between a load and an AC power source. In this arrangement, the second pilot relay is not connected in parallel with a commutation and turn-on assistance diode so that the arrangement does provide a positive circuit break in the form of an air gap between the contacts of the pilot relay between a load and an AC supply source in conformance with U.L. requirements for such switching devices. However, the system described in U.S. Pat. No. 4,296,449 is not designed to operated as a zero crossing synchronous AC switching system, and it is not known at what point in the cycle of an applied alternating current supply potential, opening or closure of the relay contacts takes place. This is due in a great measure to the slow response characteristics of electromagnetic relays generally and to the further fact that EM relays experience shifts in magnetic material characteristics, heat and age related changes, contact surface and air-gap changes and changes in the manner of movement of the relay armature resulting from the combined effect of all of the above-noted factors. Attempts to force the EM relay to obtain faster response speeds serves to increase the magnitude of these effects. An EM atuated circuit interrupter for interrupting AC currents synchronously with the passage through zero value of the AC current is described in a textbook entitled "Electrical Contacts" by G. Windred, published by MacMillan and Co., Ltd. of London, England, copyrighted 1940, see pages 194 through 197. Such a device operates to interrupt only and cannot be used for closing to initiate AC load current flow synchronously. While there may be some EM operated relays which can be used for synchronous closing of AC switch contacts, but they are not known to the inventors. Thus, zero crossing synchronous AC operation for the opening and closing with EM relay actuated switching devices is not feasible with state of the art EM relay devices.
Making and breaking current flow through a set of electric load current carrying switch contacts is a relatively complex event in the microscopic world of the forces and effects occurring at the time of contact closure and/or opening as explained more fully in the textbook entitled "Vacuum Arcs--Theory and Application"--J. M. Lafferty--editor, published by John Wiley and Son--New York, N.Y. and copyrighted in 1980. Reference is made in particular to Chapter 3 entitled "Arc Ignition Processing" of the above-noted textbook which chapter was authored by George A. Farrall, a co-inventor of the invention described and claimed in this application. From this publication it is evident that contacts of a load current carrying electric switch when overloaded, or after extended operating life, are subject to the possibility of thermal run-away which can lead to contact welding and/or creation of a fire. This can occur even though the contacts are operated perfectly during use and perform only a current carrying function. Even under conditions where there is no substantial current flow across the contacts, opening and closing of the contacts under conditions where a high operating voltage exists across the contacts, causes mechanical wear and tear so that the actual gaps between the contacts at the time of current establishment and/or extinction can change due to the effects of sparking and arcing. Thus, the long term operating characteristics of the switch contacts of a EM relay operated switch such as that described in U.S. Pat. No. 4,296,449 and other similar systems which open or close switch contacts under high voltage stress, can and do change after a period of usage.
Zero current synchronous AC switching circuits employing semiconductor switching devices such as SCRs, triacs, diacs and the like, have been known to the industry for a number of years. This is evidenced by prior U.S. Pat. No. 3,381,226 for "Zero Crossing Synchronous Switching Circuits for Power Semiconductors"--issued Aug. 30, 1968, Clifford M. Jones and John D. Harnden, Jr.--inventors, and U.S. Pat. No. 3,486,042 for "Zero Crossing Synchronous Switching Circuits for Power Semiconductors Supplying Non-Unity Power Factor Loads"--D. L. Watrous, inventor--issued Dec. 23, 1969, both assigned to the General Electric Company. Zero current synchronous AC switching circuits are designed to effect closure or opening of a set of load current carrying switch contacts (corresponding to rendering a semiconductor switching device conductive or non-conductive, respectively) at the point in the cyclically varying alternating current waves when either the voltage or current, or both, are passing through their zero value or as close thereto as possible. This results in greatly reducing the sparking and arc inducing current and voltage stresses occurring across the switch contacts (power semiconductor switching device) as the contacts close or open (corresponding to a power semiconductor device being gated-on or turned-off) to establish or interrupt load current flow, respectively. While such zero current synchronous AC switching circuits employing power semiconductor switching devices are suitable for many applications, they still do not meet the U.L. requirements of providing an open circuit gap between a current source and a load while in the off condition. Instead, while off, power semiconductor switching devices provide a high resistance current leakage path between a current source and a load. This is due to the inherent nature of power semiconductor switching devices. Again, their failure mechanism is non-fail safe. Additionally, it should be noted that the known prior art zero crossing synchronous AC switching circuits employing power semiconductor switching devices have response characteristics that are substantially instantaneous in that they turn-on or turn-off within a matter of microseconds after application of a turn-on or turn-off gating signal to the power semiconductor switching device. Hence, due to their fast responding nature, the known zero crossing synchronous AC switching circuits employing power semiconductor devices are unusable with mechanically opened and closed switch contact systems such as are used in the present invention.